A difunctional benzoxazine (coPh-apa) with a conjugated alkyne group is synthesized by the oxidative coupling reaction from a monocycle-benzoxazine (Ph-apa) containing an alkyne group. A model compound, 1,4-diphenylbutadiyne (coPa), is used to study the curing reaction process of coPh-apa by DSC, Fourier transform infrared spectroscopy, and C-13 NMR, and the results suggest that the conjugated alkyne groups are involved in the crosslinking reaction via the trimerization reaction of the conjugated alkynyl groups and the Diels-Alder reaction. Furthermore, thermal properties of the polybenzoxazine are studied by dynamic thermomechanical analysis and thermogravimetric analysis. A glass-transition temperature (T(g)s) of as high as 412 degrees C and a char yield of 75.6% at 800 degrees C under nitrogen are obtained with the aid of the conjugated alkyne groups. Its excellent heat resistance dominates most thermosetting resins and will serve for heat shields. (c) 2019 Wiley Periodicals, Inc.